You can add it under Buffs > Custom.
In the "New Buff" box, enter the spell ID, which can be found on wowhead.com if you don't have an addon that includes spell IDs in your mouseover tooltip. The spell ID for Adaptive Swarm is 325727. On that same page, you can select the priority, which controls things like where in the row of buff icons it shows up.
I can't test whether it actually shows the buff properly, since I don't have it, but I did test adding it to the custom buffs and it showed up for me. Hope that helps!